1. Field of the Invention
The present invention relates to an improved vehicle antenna system which can efficiently detect broadcast waves received by the vehicle body and transmit them to various onboard receivers.
2. Description of the Prior Art
With modern automobiles, antenna systems are essential for positively receiving broadcast and/or communication waves at various onboard receivers such as radios, TV's, car-telephones and others. Antenna systems also have an important role in citizen band tranceivers as providing a communication between an automobile and the other stationary or movable station. In future, such vehicle antenna system will increasingly be important for vehicles standardized with various receivers.
A pole type antenna is known as one of the conventional vehicle antenna systems. The pole antenna projects exteriorly from the vehicle body and exhibits a favorable performance of reception in its own way. However, the pole antenna was always an obstruction in the designs of the vehicle body.
The pole antenna also is disadvantageous in that it may accidentally or intentionally be subjected to damage and in that the pole antenna may produce unpleasant noises during running at high speeds. Therefore, it was very advantageous to eliminate the pole antenna from the vehicle body.
Recently, the number of frequency bands for broadcast or communication waves to be received by automobiles are being increased. If a plurality of pole antennas are located on a vehicle body matching the increased number of broadcast waves to be received, they would severly damage the aesthetic concept of the vehicle appearance. Furthermore, there will be created electrical interference between the pole antennas, thereby remarkably degrading the reception performance.
Some attempts have been made to eliminate or conceal pole antennas. One such attempt is application of an antenna wire to a rear window glass on a vehicle body.
Another attempt is to detect surface currents which are induced on the vehicle body by broadcast waves. Although this was considered to be the most positive and efficient means, experiments have showed that the detection of surface currents on the vehicle body is not so advantageous as expected.
The first reason is that the level of surface currents is not so high as expected. The prior art attempted mainly to detect surface currents flowing on the roof panel of the vehicle body. However, the level of surface currents is insufficient for the surface currents to be utilized as output signals of the antenna system.
The second reason is that the surface currents will contain noise signals at a very high rate. Such noise signals are produced at engine ignition systems and battery charging regulators. Therefore, such noise signals cannot be eliminated when the engine is running.
Even under such disadvantageous situations, some proposals have been made to utilize surface currents induced on the vehicle body by broadcast waves. For example, Japanese Patent Publication Sho No. 53-22418 discloses one such proposal wherein electric insulation is formed on the vehicle body at a location in which surface currents flow concentrically. Currents flowing between the opposite ends of the electric insulation are detected by a sensor. Although such a proposal provides practicable detection signals which are superior in S/N ratio, it requires a pickup construction which is not applicable to normal mass production for some reasons such as the provision of notch on a portion of the vehicle body and so on.
Japanese Utility Model Publication Sho. No. 53-34826 discloses another proposal providing an antenna system which comprises a pickup coil for detecting currents on a pillar of the vehicle body. This proposal is advantageous in that the antenna system can completely be housed within the vehicle body. However, it is not practical to provide the pickup coil disposed near the pillar and extending perpendicular to the length thereof. In addition, such pickup arrangement cannot obtain practicable antenna outputs and appears to be only an idea.
Thus, the prior art antenna system for detecting surface currents induced on the vehicle body by broadcast waves were not necessarily successful. Particularly, the prior art does not solve problems associated with the pickup construction for efficiently detecting surface currents induced on the vehicle body by broadcast waves and the pickup arrangement for obtaining practicable S/N ratios.
In addition, the prior art antenna systems of the pickup type had reduced sensitivities of reception and particularly irregular sensitivities different from one frequency band to another.